To investigate the mechanisms of human T-cell reconstitution following allogeneic hemopoietic stem cell transplantation (alloSCT), we analyzed the clonal composition of human cytomegalovirus (HCMV)-specific or Epstein-Barr virus (EBV)-specific CD8+ T cells in 10 alloSC transplant recipients and their donors. All virus-specific CD8+ T-cell clones isolated from recipients after alloSCT contained DNA of donor origin. In all 6 D+/R+ sibling alloSCTs from seropositive donors into seropositive recipients, donor virus-specific clones transferred in the allograft underwent early expansion and were maintained long term in the recipient. In contrast, in 2 of 3 HCMV D+/R- alloSC transplant recipients in whom there was no detectable HCMV infection, donor HCMV-specific clones were undetectable, whereas donor EBV-specific clones were maintained in the same EBV-seropositive recipients, suggesting that transferred clones require antigen for their maintenance. Following D-/R+ transplantation from 3 seronegative donors into seropositive recipients, a delayed primary virus-specific CD8+ T-cell response was observed, in which the T cells contained donor DNA, suggesting that new antigen-specific T cells arose in the recipient from donor-derived progenitors. In 2 of 4 HCMV D+/R+ sibling allograft recipients the clonal composition underwent diversification as compared with their donors, with delayed persistent expansion of HCMV-specific clones that were undetectable in the donor or in the recipient during the early months after transplantation; this diversification may represent expansion of new clones generated from donor-derived progenitors. We conclude that, following alloSCT, late diversification of the HCMV-specific CD8+ T-cell clonal repertoire can occur in response to persistent viral antigen.